One of the problems involved in purifying gas mixtures such as gas mixtures containing hydrogen, saturated and unsaturated hydrocarbons, carbon monoxide and organic sulfides, e.g., mercaptans such a methyl and ethyl mercaptan, carbonyl sulfide (COS) and carbon disulfide (CS.sub.2) resides in the fact that there are a number of competing chemical reactions which can occur and which are not compatible with one another. Thus, carbon monoxide can be removed by reaction with water to form hydrogen and carbon dioxide in what is called the water-gas shift reaction. Olefins such as ethylene can be eliminated by saturation with hydrogen. Organic sulfur contaminants such as carbonyl sulfide (COS), methyl and ethyl mercaptan and carbon disulfide are rather difficult to remove from such gas mixtures. Another reaction which occurs in the methanation reaction wherein one mole of carbon monoxide reacts with three moles of hydrogen to produce one mole of methane and one mole of water. This reaction occurs with subtantial increases in temperature so that it is difficult to control, especially if it occurs simultaneously with other reactions.
A well known carbon monoxide shift catalyst is an iron oxide-chromium oxide catalyst. Metallic iron is undesirable in this catalyst because it catalyzes the methanation reaction and also a reaction whereby two molecules of carbon monoxide break down to form one molecule of carbon dioxide and one molecule of carbon. Some catalysts such as Fe.sub.2 O.sub.3 promoted with Cr.sub.2 O.sub.3 are sulfur resistant. Other catalysts such as copper deposited on zinc oxide which can be used in the carbon monoxide shift reaction will not tolerate even traces of sulfur.
U.S. Pat. No. 3,850,840 discloses a process for preparing hydrogen and carbon dioxide by contacting carbon monoxide and steam with a chromium oxide promoting ferric oxide catalyst comprising an alkali metal compound and a hydrogenation-dehydrogenation component comprising vanadium, molybdenum, tungsten, cobalt, tantalum, or niobium materials or mixtures thereof, or mixtures of materials comprising a vanadium, molybdenum, tungsten, cobalt, tantalum or niobium material or mixtures thereof with a nickel, iron or chromium material or mixtures thereof.
Another patent disclosing a process for preparing hydrogen and carbon dioxide with similar type catalysts is U.S. Pat. No. 3,850,841.
These patents for the most part are concerned with relatively simple gas mixtures in which the major components are hydrogen and carbon monoxide and minor components are carbon dioxide, nitrogen, methane and hydrogen sulfide. The principal reaction involved is the conversion of carbon monoxide to hydrogen and carbon dioxide so that there is no substantial problem with regard to competing reactions where organic sulfur contaminants are present or where unsaturated hydrocarbons such as olefins are present.
The problem of preparing hydrogen suitable for use in hydrotreating petroleum hydrocarbons or for the manufacture of ammonia, or for other purposes where hydrogen of high purity is desired, becomes more complicated when the original source of the hydrogen is one which also produces olefins such as ethylene and organic sulfur compounds of the type previously mentioned as well as carbon monoxide, all of which have to be removed before the hydrogen is suitable for the aforesaid intended purposes. As an example, gas mixtures of this type are produced by heating shale under anaerobic conditions. It is theoretically possible to remove the unwanted impurities by a series of steps in which each of them is removed in a separate step but it would be highly desirable, if possible, to remove or reduce the unwanted impurities such as carbon monoxide, olefins and organic sulfur compounds in a single step while at the same time controlling the reactions involved so as to avoid uncontrollable temperature conditions.